Bruce, your sleuthing is amazing! Right up there with Gene and the 60 amp. ammeter. I'm very impressed! One difference between the part you've shown and the original may be the material. I'm guessing that the original was aluminum as indicated by the "D" after the 1/4. In contrast, your example is made of steel and has an "S" after the 1/4. Doing a search I found another part number which supersedes the original. Part number 0550100-10. KRN has one.
Now regarding whether this check valve has male or female threads. The IPC shows two variations for how this valve is plumbed; in the top right corner on Fig. 34 is the early version, (s/n 18001-18220) as shown here.

- Check Valve.jpg (45.9 KiB) Viewed 29674 times
At the inlet to the check valve is an unlabeled tee. Clearly though it has a male pipe fitting which screws into the check valve. This would appear to be an AN825-6D.

- AN825-6D
- AN825C.jpg (109.06 KiB) Viewed 29674 times
At the outlet of the check valve is a AN911-2 nipple.

- AN911-2
- AN911C.jpg (1.94 KiB) Viewed 29674 times
The bottom right corner of Fig 34 illustrates the later setup, (s/n 18221-18729):

- Later Style.jpg (47.11 KiB) Viewed 29674 times
Notice that the AN826-6D tee has the male pipe on the run which screws into the check valve.

- AN826-6D
On the downstream side of the check valve there is no nipple. The male pipe thread of the cross screws right into the check valve.